block chain analysis

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Blockchain Technology for Cloud Storage: A Systematic Literature Review

PRATIMA SHARMA and RAJNI JINDAL, Delhi Technological University, Delhi, India MALAYA DUTTA BORAH, National Institute of Technology Silchar, Assam, India

The demand for Blockchain innovation and the significance of its application has inspired ever-progressing

exploration in various scientific and practical areas. Even though it is still in the initial testing stage, the

blockchain is being viewed as a progressive solution to address present-day technology concerns, such as de-

centralization, identity, trust, character, ownership of data, and information-driven choices. Simultaneously,

the world is facing an increase in the diversity and quantity of digital information produced by machines and

users. While effectively looking for the ideal approach to storing and processing cloud data, the blockchain

innovation provides significant inputs. This article reviews the application of blockchain technology for se-

curing cloud storage.

CCS Concepts: • General and reference → Surveys and overviews; • Computer systems organization

→ Peer-to-peer architectures; • Information systems → Distributed storage; • Security and privacy →

Security services;

Additional Key Words and Phrases: Blockchain technology, decentralization, cloud computing, cloud security,

cloud storage

ACM Reference format:

Pratima Sharma, Rajni Jindal, and Malaya Dutta Borah. 2020. Blockchain Technology for Cloud Storage: A

Systematic Literature Review. ACM Comput. Surv. 53, 4, Article 89 (August 2020), 32 pages.

https://doi.org/10.1145/3403954

1 INTRODUCTION

In recent years, cloud technology has attained an emerging trend by showing a possibility in both academia and industry for its efficiency and availability. Although it is a widely accepted technol- ogy, with an outburst of data origins, there has been an increased issue of storage and usage of data owing to the inability of a conventional data management tool to manage the exponentially growing data. The traditional cloud storage model comprised a back-end platform that could be storage or server, a front-end platform that could be a mobile device or a client and a network, possibly an intranet or internet. Attention is given to cloud technology with this outburst of data origin by the researchers in presenting solutions for the intricacies of storage and usability in cloud storage technology [1].

Author’s addresses: P. Sharma and R. Jindal, Department of Computer Science and Engineering, Delhi Technological Uni-

versity, Bawana Road, Delhi-110042, India; emails: [email protected], [email protected]; M. D. Borah,

Department of Computer Science and Engineering, National Institute of Technology Silchar, Fakiratilla, Silchar, Assam

788010, India; email: [email protected].

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https://doi.org/10.1145/3403954

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Cloud computing is usually adopted in military and commercial environments to aid in data storage. The heterogeneous environments of cloud computing are distributed with various com- ponents of the hardware and software that are obtained from the vendors, which could introduce incompatibility and vulnerabilities. The security affirmation of inter- and intra-cloud transfer and management of information emerges as a key issue [2]. Cloud computing is a pay-per-use model to enable on-demand, convenient available network access for shared computing resources, such as storage, servers, applications, and networks, which could be provisioned quickly and released with minimum effort of management or interaction with the service providers [3, 4].

Outsourcing computation is a vital service provided by cloud computing, thereby acting as one of the significant advantages of the cloud. With the approach of pay-per-use, the computing re- sources are used by the cloud, further overcoming the limitations of computationally weak devices [5, 6] by outsourcing their data within the cloud. The user can rent and pay the storage services or utility computation based on the requirement with the help of the cloud services. When compared with traditional storage techniques, the cloud is more flexible and has elasticity [7]. The user device has a limited storage capacity, so the data are stored on the cloud. For the classifier and data, con- fidentiality is vital because of privacy requirements, and the service provider is not trusted [8, 9].

The major challenge explored and highlighted by the researchers is the processing and storing of data into the cloud. Moreover, while saving the data, the issue of heterogeneity is considered the main challenge for the researchers. This issue of heterogeneity in data storage is termed as big data or large-scale data. The cloud environment and the technology of the blockchain [10] are adapted for this usability. Therefore, for the improvement in the performance of existing appli- cations, these two approaches are combined [11]. Therefore, blockchain is a distributed and safe networking system in which many computers called nodes are stored. This technology is of the utmost importance with many possibilities for transmitting and storing a vast volume of data. It also minimizes costs and improves accuracy [12].

In this survey article, the following main points are covered:

• An overview of cloud storage and blockchain technology is briefly highlighted and analyzed.

• Existing cloud storage techniques and blockchain-based studies are addressed, and their advantages, disadvantages are explained.

• Various blockchain-based cloud storage applications are explored. • Blockchain-based distributed cloud storage technologies are analyzed based on various pa-

rameters, and basic building blocks are summarized. • Inferences and recommendations are given.

Figure 1 illustrates an overview of the study in the form of a block diagram. First, the article presents an overview of cloud computing and blockchain as separate concepts, reviews the studies that integrate blockchain and cloud storage, and analyzes real-life projects based on blockchain and cloud storage. Finally, some properties of blockchain for secure cloud storage are analyzed, chal- lenges are discussed, inferences are drawn from the study, and some suggestions are made based on the inferences. This article is arranged as follows: Section 2 discusses the procedure for the sys- tematic literature review. Section 3 covers the background details of cloud storage and blockchain technology. The work-related to cloud computing and blockchain technology is explained in Section 4. Section 5 covers the study of current blockchain-based cloud storage. Section 6 includes the various real-life projects related to blockchain-based decentralized cloud storage technolo- gies. Section 7 presents the properties of blockchain-based secure cloud storage. Blockchain and cloud storage integration requirements and challenges are listed in Section 8. Section 9 highlights

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Fig. 1. Overall flow of the study.

inferences and suggestions related to blockchain-based cloud storage. Finally, conclusions are pro- vided in Section 10.

2 METHODOLOGY OF SYSTEMATIC LITERATURE REVIEW

One of the main research areas of research methodologies is the Systematic Literature Review (SLR). The undertaking of the SLR methodology is crucial for evaluating current cloud storage details. The main aim of this review article is to examine or detect the relevant literature based on blockchain for cloud storage. The SLR process phases are illustrated as follows:

As shown in Figure 2, the SLR phases include review protocol, searching queries, selection of sources, study selection, data extraction, and analysis. Review protocol sets out the strategies that are to be used in the process of a systematic review. In the current research, the SLR’s principal methodology is used to discover the published papers in the field of blockchain technology and cloud storage. This study’s main objective is to provide an overview of the current research on blockchain-based cloud storage techniques. Thus, we explained six research queries.

• RQ1: What are the different prevention methods utilized by cloud storage providers while sharing information to anticipate the threats?

To identify the present security methods used in cloud computing.

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Fig. 2. Overview of SLR process.

• RQ2: What are the different strategies used for preventing the illegal use of data stored on the cloud?

To identify the different techniques that are used for preventing the stored information in the cloud.

• RQ3: What are the main security difficulties to be faced in the future based on cloud storage technology?

To discuss future cloud computing security techniques.

• RQ4: What are the current research topics of blockchain technology?

To study and understand the blockchain technology, we thereby collected all the critical research papers from logical databases and mapped the current research area.

• RQ5: What are blockchain technology’s current research topics for cloud storage?

To study and understand the blockchain technology-based cloud storage techniques. This would, hence, help the researchers to figure out the present research topics related to blockchain technology for cloud storage.

• RQ6: What are the different methods utilized to save, delete, and update the cloud data using blockchain technology?

To study and understand the methods used to perform the various operation on the cloud data using blockchain technology.

The search was directed from 2010 until 2019 by utilizing the online logical database. The search methodology for the survey was necessarily coordinated toward finding distributed papers in jour- nals and conference papers through the accepted literature search engines and databases Google Scholar, Springer Digital Library, ACM Digital Library, IEEE Xplore, Elsevier and Science Direct. In 2018, the published articles had the highest rank. Likewise, papers distribution based on published articles from 2010–2019 appears in Figure 3.

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2 3 2 2 5 5

15

39 40

22

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 0

10

20

30

40

50

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ef er

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Fig. 3. Distribution of papers from 2010 to 2019.

3 BACKGROUND

3.1 Overview of Cloud Storage

The cloud is described as the chain of servers and connections to give a computing benefit for storing the user data. Presently, several organizations and internet sources are adopting cloud storage for individual and organizational users [13]. One of the cloud computing models is cloud storage, which stores a colossal amount of data and can be retrieved using the internet. Cloud storage service providers can control any data, i.e., organized or semi-organized [14].

Cloud storage provides a deliberation to physical capacity gadgets. Also, it presents information storage as a service, regularly charged on a user premise. It is this reason for cloud storage, which enables a client to store and access information records someplace within the cloud, without un- derstanding the points of interest of where documents are saved. Moreover, the record can be made accessible on a worldwide premise in cloud storage [15]. The expansion of the number of clients who put their benefits on the cloud has led the cloud storage to become an important theme. How- ever, these clients regularly do not trust the fact where their information will be stored and who will approach this information. Hence, numerous clients feel the commitment of applying safety efforts to have an aggregate authority over their information. For this, authentication, integrity, availability, confidentiality, and privacy problems are the requirements that the users look for. In the explicit instance of endeavor, these suggest essential contemplation; ought to be incorporated into any cloud benefit contract.

3.2 Blockchain Technology Overview

Blockchain is one of the most hyped advances nowadays and has gained considerable importance as an innovation widely deployed in various areas [16, 17]. The blockchain is viewed mainly as an accounting book or digital distributed database [18]. After its commencement in 2008 [19], blockchain has continued to develop as a disruptive advance that might alter the way we inter- face, make computerized expenses, follow up, and monitor transactions [20]. Blockchain could be cost-effective, removing the centralized authority’s need to monitor and regulate transactions and interactions between different members. In the blockchain, every transfer is cryptographically marked and confirmed by other mining entities holding a copy of the entire record consisting of all the transactions. This makes records step by step, safe, synchronized, and shared time that cannot be adjusted [21]. Furthermore, blockchain technology is known to be an information technology that can be used in software, business, and trade sectors [22]. The architecture of the blockchain is illustrated in Figure 4.

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Fig. 4. Architecture of Blockchain.

Key Characteristics of Blockchain

• Distributed: The distributed environment uses the standard protocol, which ensures that ev- ery node receives each transaction and uses predefined rules for grouping the transactions into blocks after processing. The blockchain is designed for distributing and synchronizing the data across multiple networks.

• Decentralization: It is the core strength of blockchain, because each node holds a record of all transaction data, so there is no need for a central authority. This relief the failure of the single point of vulnerability. In the blockchain network, there is no single authority and no service fees, and the consensus algorithms are used to maintain the data consistency.

• Consensus: Consensus algorithms maintain the consistency of data within the blockchain and keep incorrect or false transactions away from the blockchain network [23]. All nodes must agree by executing the standard consensus algorithms to ensure the integrity of trans- action data. Moreover, there must be an assertion between every member, before one can execute the transaction, only then the transaction should be valid. This procedure is called a consensus [24, 25].

• Anonymity: In the blockchain, every user can interact with a created address. The system will not disclose the user’s actual details; however, the members can view the encoded trans- action details.

• Traceable: The blockchain is time-stamped and digitally signed, which implies that the as- sociation can follow back to an explicit time for every transaction and further distinguish the relating party on the blockchain [26]. Consequently, each block is permanently and unquestionably connected to the past block [27].

4 RELATED WORK

This segment presents research works relating to traditional cloud computing techniques that proved their significant improvement toward increasing security and academic studies on the blockchain and cloud computing.

4.1 Traditional Cloud Computing Techniques

Cloud storage is a kind of Internet technology for sharing resources with IT-related capabilities, and it is important to either enterprises or individual users. Traditional security strategies mainly focus on information encryption [28, 38], integrity checking [28–34, 36], data deduplication [30, 39, 40], user revocation [35], data storage [38], data auditing [37], and so on. While Information

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and Communications Technology (ICT) and cloud services progressed, numerous scientific work has been dedicated to increasing the performance and safety of the data. To protect user data stored on the cloud, a symmetric cryptographic scheme is used with encrypted bloom filters to al- low the user to recognize unauthorized modifications in the outsourced data [28]. In 2018, Yunxue Yan et al. [29] developed a protection scheme to enhance the security of signature information for user data. In [30], the authors proposed an integrity assurance algorithm using a standard storage template for various control methods. Tags for validation and process generating proofs relied on the index pointers. In References [31–34], the authors introduced a novel and efficient integrity verification technique by using various approaches like Merkle hash tree, proof of storage, and paillier homomorphic cryptography method, respectively. In References [35, 37], the authors il- lustrated a safe group user revocation method with an effective public integrity audit scheme. It facilitated public monitoring and the effective removal of users. Yibin Li et al. [38] concentrated on the problems of cloud operator misuse concerns aimed to prevent the cloud user’s data re- lease from cloud servers. Pooranian et al. [39] proposed a RAndom REsponse (RARE) method that removed the cloud storage facilities deduplication response side channel and retained the use of deduplication simultaneously. Chia-Mu Yu et al. [40] proposed a zero-knowledge deduplica- tion response as a side-channel shield that is based on a zero-knowledge cross-user deduplication response structure. Although these schemes provide secure storage, integrity checking, efficient user revocation, and data duplication removal, there are still some problems existing in the sys- tems, such as centralized data storage that severely harms physical server security and the need for trusted third-party, which are nightmares for the privacy of users’ data. It has been observed that most methods consider only static data sets and does not apply to a large volume of data. The issues of cloud storage data security are not resolved through some techniques. Therefore, it seems very meaningful to understand the blockchain-based solutions for cloud storage known so far and to carry out investigations to conclude.

4.2 Blockchain and Cloud Computing Studies

A lot of new technologies and frameworks have been introduced with the existing keen interest in blockchain technology. Numerous review articles were published to demonstrate the advantages of blockchain for existing applications. Examples of these studies include the blockchain tech- nology for business applications [41, 42], e-governance [43], healthcare [44–46], security [47–48, 52, 73], sharding [49], cloud exchange [50], edge computing [51], and so on. Certain studies dealt with blockchain obstacles, prospects, and plans for the future. For example, References [47, 52, 53] address security challenges and opportunities in the blockchains. The research in [81] pro- vides a detailed overview of privacy and security concerns in cloud computing, covering potential threats and detection methods based on blockchain. Authors address in References [45, 46] the use of blockchain technology in the healthcare and medical fields. Reference [49] presents key concepts of various sharding mechanism focused on blockchain technology. In Reference [50], the authors address issues in terms of safety, security, and transaction processing regarding the use of blockchain for cloud exchange. Moreover, Reference [51] is dedicated to blockchains for edge computing systems and their potential uses. All the earlier studies under consideration dis- cuss the safety pattern in cloud storage and plan to incorporate blockchain technology in different environments. This article investigates the use of blockchain technology for cloud storage as a whole without being specific to particular applications, thus addressing its current trends, clas- sifications, and open issues that have not been discussed in the prior surveys. We are aiming to provide a detailed overview of the usage of blockchain technology in cloud computing. To the best of our understanding, our research surpasses all the current studies more systematically in terms of the core principle of blockchain technology for cloud storage.

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Fig. 5. Blockchain methods selected in this SLR.

5 EXISTING TECHNIQUES OF BLOCKCHAIN FOR CLOUD STORAGE

This area gives the details of the blockchain methods selected in this SLR. Based on selected studies, we arranged the blockchain techniques utilized for cloud storage as follows:

• Cryptography Technique (CT) • Data Deduplication Scheme (DDS) • Data Integrity Checking Technique (DIC) • Storage Efficiency Technique (SET) • Bitcoin Technology (BT) • Blockchain-based Cloud Storage (BCS)

In Figure 5, we have used the following abbreviations to represent methods selected in this SLR: FAC, Fine-Grained Access control; DA, Data Auditing; SE, Searchable Encryption; VC, Verifiable Computation; RDD, Reliable Distributed Deduplication; SDD, Secure Data Deduplication; BT, Bit- coin Transaction; NG, Non-repudiation Guarantee; MT, Merkle Tree; ET, Ethereum Technology; HF, Hyperledger Fabric; HT, Hash Technology; DS, Digital Signature; PW, Proof of Work; PM, Payment Method; DS, Deletion Scheme; ACS, Access Control System; ES, Encryption Scheme.

5.1 Blockchain-based Cryptographic Technique for Cloud Storage Service

The framework allows the users to transfer their information in encoded form, distribute the infor- mation substance to cloud hubs, and ensure the accessibility of information using cryptographic procedures [55].

The structure of cryptography storage contains three parts, as shown in Figure 6.

• Data Processor (DP): Processing of information before sending it to the cloud. • Data Verifier (DV): Verification of the damaged information stored in the cloud. • Token Generator (TG): For saving the documents of the clients on the cloud, the token

generator generates the token for each user.

Figure 6 represents: (1) Master “y” information processor sets the information before sending it to the cloud; (2) Master “x” needs the permission of Master “y” for scanning a keyword; (3) Master

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Fig. 6. The architecture of cryptographic cloud storage.

“y” generates a token for a keyword and send it back to Master “x”; (4) Master x receives the token and send it to the cloud; (5) To locate the appropriate encoded documents, the cloud utilizes the token and send the resultant documents back to Master x. (?) At any time, Master y’s data verifier can confirm the integrity of the information.

5.2 Blockchain-based Data Deduplication Scheme for Cloud Storage Service

The data deduplication scheme is used to eliminate the superfluous data and to optimize the storage space in the cloud. Additionally, this technique only retains one copy of the indistinguishable information to optimize storage capacity. Thus, it can manage data efficiency and save the cost of physical devices [56], but there is a chance of increasing data reliability problem. Thus, the files are distributed to different servers using a deduplication approach, and the storage information is recorded on the blockchain. The protection of system confidentiality and data integrity can be achieved by combining the data deduplication scheme with the blockchain technique. Also, it is well-suited for distributed storage systems. The blockchain network should be joined by CSP and data owner as a node for related services. Every duplication and transaction data should be documented on blockchain for ensuring the data authenticity [57]. The data deduplication method is classified based on Data unit, Location, and Disk placement [58].

The deduplication process is categorized into three methods based on where the data are pro- cessed: data unit deduplication, location deduplication, and disk placement deduplication. The classification of deduplication is illustrated in Figure 7. The data unit deduplication is bifurcated further into file-level and chunk level deduplication. The file deduplication uses the unique hash values for comparing the two files. If the hash values are identical, then only one copy is stored. In deduplication at the chunk level, the file is split into fixed or variable length blocks and then check for the duplicate content. Location-based deduplication is categorized into source and tar- get deduplication process. The target deduplication process is worked at the side of a receiver after the client transmits the files, and it also rejects the additional data. Without influencing the client’s operation, the deduplication process is done by the storage device. The process of dedupli- cation is hidden from users. Before transmitting the data, the source deduplication process is done. This deduplication has the advantage of network traffic bandwidth, because it utilized the client’s

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Fig. 7. Classification of deduplication.

resources. The disk-level deduplication is further divided into two parts; these are forward refer- ence deduplication and backward reference deduplication.

5.3 Blockchain-based Data Integrity Checking Technique for Cloud Storage Service

The decentralized data integrity checking is enabled by a blockchain-based structure for cloud storage services. The integrity checking method is mostly dependent on three components. They are the blockchain, the Cloud Storage Service Provider (CSS), to which the information is out- sourced and the data user. Data Integrity Service (DIS) is built on the structure of the blockchain. The blockchain client should be started initially on nodes when the DIS is needed [59]. Blockchain can use Merkle trees for data integrity verification. This comprises two phases: the pre-processing stage and the validation phase.

• Pre-processing phase: Initially, the data are sliced by the data consumer to form different shards, and these shards are then used for constructing a hash Merkle tree. Then, the con- sumer and CSS will approve the hash Merkle trees, and the consumer stores the root of this hash tree. The user data and public Merkle trees are uploaded on CSS. CSS sends the address of the stored user’s data to the user.

• Verification phase: CSS receives a challenge number from the client, and it is used to select shards to check. The hash function is used based on challenge number and shard to cal- culate a hash digest. CSS forward digest and the equivalent are supporting statistics to the blockchain. The smart contract calculates a fresh hash root and compares the hash roots. The data integrity will be assured when the hash roots are equal. If not, then data integrity has been degraded. Finally, the verified output is sent to the client by the blockchain [60].

5.4 Storage Efficiency Technique for Cloud-based Design

With the rapid increment of cloud computing, the NoSQL database is the best choice for saving the information into the cloud. The NoSQL databases are stored and handled by platforms like MongoDB and Hadoop, Graph Databases, Column-oriented DBs, Document DBs, and Key-value stores [61]. The cloud stored data in a simple text format; hence, it is a very inefficient way of storing data. This makes some issues in the cloud and further raises the overhead of operating

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system while storing the data. To manage huge volumes of data, the MapReduce method is used, but it is not suitable for a relational database management system [62]. BigchainDB is a masterless, scalable, decentralized database of cloud data storage. BigchainDB is incorporated on top of a NoSQL RethinkDB database as an additional layer, which is efficient storage for cloud storage. It utilizes a database underlying and provides blockchain-like functionalities such as hashed blocks, transactions, voting, and record immutability [63].

5.5 Redundant Array of Cloud Storage (RACS)

For distributing the data and protecting the network architecture, RACS is used. RACS is per- formed between the customer and various data repositories [64]. It showed parallel communica- tion with multiple proxies in a distributed manner. It can likewise be kept running on various intermediaries with a similar arrangement of the vault utilizing strategies. This strategy is primar- ily acquainted with maintaining a strategic distance from seller secure and with decreasing the expense of exchanging suppliers. The supplier disappointments are endured. The system is basic and straightforward. Since all information must go through a RACS intermediary either to encode or to decipher, a solitary intermediary could without much of a stretch turn into a bottleneck [65]. The users can utilize blockchain-based secured cloud storage architecture [66], where they would split their documents into several chunks of data and randomly upload those chunks of data to the P2P network offering free storage capacity.

5.6 Blockchain-based Cloud Storage Access Control System

Blockchain technology may be utilized to provide secure access control to the information col- lected in an untrustworthy cloud environment. The untrusted cloud storage environment needs the technique to protect the shared data. Blockchain-based access control provides a method using the attribute-based encryption scheme with dynamic attributes for user access [67]. Using the de- centralized ledger technology, blockchain keeps the unchanged log for all relevant security events, like revocation, key generation, access policy appointment, and access request.

A blockchain-based access control technique is developed in Reference [68]. Access control is inclusive of identifying, authenticating, and authorizing procedures. It ascertains the state of being responsible wherein client access can be tracked for which specific activity in a framework. The introduced framework allows the clients to get to Electronic Health Record (EHR) from the data pools that are shared utilizing blockchain in the wake of checking their cryptographic keys as well as their identity. To accomplish the client’s authentication, validation based on identity is adopted. Banning the malevolent imitation of roles and adaptability, which effectively allows organizations to participate and consumers from regulating their responsibilities. To guarantee secure access to sensitive information, creating a smart contract-based access control mechanism is intended to be reliable, flexible, and useful. The Smart Contract-based Role-based Access Control (RBAC-SC) utilizes blockchain and smart contracts as versatile systems to portray the relationship of trust that is crucial within the RBAC and for the execution of authenticating the challenge-response process, which will verify the user’s possession of positions [69].

5.7 Blockchain-Enabled Payment System for Cloud

The security attacks on the present payment framework are increasing due to its complexity and scattered nature of the transaction facilitators. A client expecting to exchange cash will pay a yearly participation expense to get the card and utilize it to buy merchandise or use the services. The banks of the client and the dealer associate with one another to settle the charge expect- ing to utilize the card obtained from the bank and used it in buying the merchandise and enter- prises. A simplified transaction is necessary as more individuals use cell phones to buy service or

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Fig. 8. The process of a payment system.

merchandise [70]. Concerning the advantages of the execution of a customer transaction as a P2P transaction utilizing blockchain, the exchange is not just dependable and unquestionable yet additionally cost-effective, since there are no outsiders included. Also, the transaction utilizing blockchain could be finished quickly as the physical distance does not influence the transaction, while regular transactions over the border could be slow. In contrast, traditional, centrally managed transactions are defenseless against significant information leakage during database management as all valuable information is handled within the central server. Conversely, it is exceptionally hard for attacking the transactions based on blockchain, since all of the vital information is conveyed, and an assailant must hack and change 51% of the shared P2P. Outsourcing services generally in- volve internet security and payment problems as an appealing business model for cloud computing. Mistrust among customers and outsourcing providers may significantly hinder the acceptance of wide-ranging cloud services. Some present payment techniques, however, only take into consid- eration an outsourcing provider and relying on the trusted third party. For ensuring a safe and fair payment service without a trusted party, a blockchain-based online payment method is needed for outsourcing cloud computing services [72].

5.8 Blockchain-based Cloud Storage Data Deletion Scheme

The cloud server maintains the users’ information to reduce the overhead of saving, updating, and deleting data. Therefore, it is necessary to incorporate the security feature while deleting, storing, and updating the cloud data. Recently, several research works [73] have been carried out to delete the desired selected data safely. Most of the available techniques, however, can be represented

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using the same “one-bit-return” protocol procedure: The cloud server removes the data and re- verses the one-bit result. The owner of the information must trust the outcome, because the owner cannot verify it. Consequently, the deletion scheme based on the blockchain can increase trans- parency in the deletion operation. The data owner may test the deletion result irrespective of how malevolently the cloud server behaves. Also, the safe deletion technique can obtain public vali- dation without any third party with the application of blockchain [73]. Even though deleting the data securely has been hypothetically explained; however, the proposed plans still have two char- acteristic constraints. The majority of the proposed plans with overwriting strategy are unable to support verification. In such protocols, the owner of the information must accept the framework of managing the data, since they cannot check the consequence of data deletion. Despite a few schemes verifying, they have to introduce a third party that can be trusted. The other innate con- straints are that the proposed conventions are not effective in the case of practical applications. It is considerable for designing secure information deletion plans to erase information effectively and permanently [74].

5.9 Blockchain-based Bitcoin Technology for Cloud Storage

Bitcoin is known as the digital currency, which relies on the P2P payment platform that has been developed as open-source software. The virtual currency is transferred and established within the cryptographic links, and bitcoin is also known as cryptocurrency [75]. Bitcoin brought about a revolution, since it gave a technique for P2P transactions without the requirement for interme- diate bodies like the banks. The bitcoin dependent on the blockchain relies on the cryptographic algorithms that are highly secure and sophisticated P2P techniques that form the fundamentals of the democratically sustained and distributed public ledger of transactions. The transactions are validated, and a permanent record is maintained by bitcoin while ensuring that the identity-related user data is kept incognito [76]. Accordingly, by applying Blockchain or comparative digital cur- rency methods, the clients neither require confiding in one another nor require an intermediator; instead, the trust is shown inside the decentralized system framework itself.

5.10 Searching Process in Blockchain-based Cloud Storage

Conventional cloud storage depends on only great-sized storage providers, who go about as third parties that can be trusted for transferring and storing data. This model represents various issues, including information accessibility, high operational expense, and information security. Various studies have presented a framework that influences blockchain technology for providing protected distributed data storage along with the service of keyword searches. The framework permits the customer to transfer the information in an encoded manner, transfers the content of data to cloud servers, and guarantees information accessibility utilizing cryptographic strategies [77]. It addi- tionally gives the owner of the data the ability to give authorization for others to search on the information. Also, the system allows scanning for private keywords over encoded data collection [78].

5.11 Auditing Scheme in Blockchain-based Cloud Storage

With the fast improvement of cloud computing, an expanding number of companies and people share and store the information on untrusted clouds. Accordingly, the auditing of shared infor- mation has become a major issue in cloud storage, pulling scientific consideration. An open audit (public audit) shared cloud storage information protocol using blockchain and Rank-based Merkle AVL tree (RB-MHT) to achieve privacy conservation and batch auditing to preserve the protec- tion of the updated blockchain record in the scheme [79]. The main thing that the TPA needs for checking the data evidence is the group manager’s public key. Furthermore, the community

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manager cannot change the changed records discretionarily. The Performance Evaluation indicates the proposed plan is safe and effective.

A smart and decentralized public auditing plan for cloud storage was proposed in Reference [80]. By bringing the blockchain framework into the plan eliminated the TPA in the framework model. Stability and reliability are enhanced because of the completely decentralized development. Likewise, they developed an automatic auditing protocol along with the smart contract, which can check periodically the integrity of the data in the cloud instead of the owner of the data. Hence this ensures that the data owner is rid of the periodic verification burden. Since every single, smart contract is executed and stored by all nodes in the system, the audit results cannot be altered.

5.12 Security and Privacy Issues in Blockchain-based Cloud Storage

A blockchain discarded the server to ban the central authority’s association and enabled transac- tions by members who saved the exchange documents collectively and finally endorsed transac- tions using the P2P network technique. The blockchain has a shared framework and makes use of peer network and peer resource computation. Specialized estimates, for example, Proof of Work (PoW) and Proof of Storage (PoS), were implemented for improving blockchain security. Even though blockchain security is continuously improving, problems have continued to be accounted for, and there are diverse safety assessments. An intruder may make various attempts to access the personal keys stored on the client’s device or cell phone. Investigations are underway on using a secure token or storing it securely to protect the personal key. In Reference [81], the authors dis- cussed the advancement of blockchain and related technologies and analyzed the research trend to identify more areas of study. The use of blockchain in the cloud computing environment should be considered for specific current problems. Even now, Blockchain contributes to numerous issues, such as transaction security, wallet and programming, and various investigations have been un- dertaken to resolve these issues. When using blockchain in the cloud computing environment, the anonymity of the client data should be assured, and the client data should be completely erased when the software is deleted. The client data can be inferred from the rest of the data in case the client data is not deleted yet slightly left behind. It appears that efficiency assessments are addi- tionally required alongside protection, given the environments in which large amounts of data are transmitted.

5.13 Encryption Methods Used in Blockchain-based Cloud Storage

Cloud computing has pulled in an expanding number of people and companies to outsource their information to third-party platforms for boundless storage and computing capacities upon request with convenience and inexpensiveness. Since cloud servers cannot be trusted and the information security of clients, it is important to encode the information before it is outsourced to clouds. But, the immediate utilization of conventional encryption advancements denies clients of searchability and, in this way, brings about a poor client experience. To safeguard this search service, accessible encoding advancements have been created in two delegate settings, including the settings of public keys and symmetric keys [82].

5.14 Blockchain Technology for Off-Chain Cloud Storage

Current-age corporations have handled information as on-chain or off-chain storage structures for blockchain-based solutions. This could be applied as data storage in a blockchain infrastructure that is private or publicly available. Off-chain does not necessarily mean “not at the ledger,” it sim- ply means it’s not on a database that is open to the public. Just as any company might not maintain the information in a publicly accessible repository or archive, off-chain management ensures the information is not available to the general public [83]. Off-chain transfers are of enormous value,

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since they have improved safety and are not constrained by the transactional speed restrictions. In a traditional on-chain transaction, each transfer would have to be validated by all peers in the network before the transfer is labelled as complete, which keeps it very slow. In contrast, in an off-chain transfer, not all peers need to wait for the transfer to be verified before it is labelled as successful or complete.

Because off-chain platforms are not linked to the public internet, it is more protected, since it is very close to the protection that could achieve by deploying a server or software in the in- tranet as opposed to the internet. The on-chain transaction includes a lot of members checking transfers, and all participants’ validation signatures have to be an exact match to be considered valid for that transaction. Whereas the details of each exchange are released for inspection on the public blockchain so that they are not altered or turned back, this may take longer than for off- chain transactions. Also, there is a very probable possibility for the payment costs to be costly, as participants can choose the off-chain scheme [84].

In an article published by IBM [85], off-chain transactions deal with “values outside of the Blockchain that can be finished using a variety of methods.” Both sides must concur on the transfer, and then another party comes in to verify the transaction. Because non-transaction information such as images, agreements, PDFs, and private data are not recommended to be stored in the main blockchain database, some kind of off-chain or sideDB space is required. A hash or signature will be produced for the off-chain object, and that is what is kept in the blockchain database. The in- dividual object is stored either in the cloud, on-premises, or in a near-cloud storage network. The needed capacity for off-chain data is expected to exceed storage requirements for the blockchain database. Any kind of off-chain transaction is pretty fast and immediate, without the higher on- chain transaction fees.

The various applications of blockchain technology for cloud storage and their properties are summarized in Tables 1 and 2. Removal of duplicate data on the cloud, storage of cloud data using blockchain, maintaining the security of the cloud data, encryption methods, security and privacy issues, safe deletion of data, secure access control mechanism, off-chain cloud storage, and fair payment method utilized for cloud services using blockchain technology are presented in these tables.

6 REAL PROJECTS: BLOCKCHAIN-BASED DISTRIBUTED CLOUD

STORAGE TECHNOLOGIES

In this section, we explore and analyze some of the popular distributed storage platforms and highlight the main contributions. Marketplaces for cloud storage make disk space a commodity. They are the intermediary with those who want to store information and providers who are willing to store data for them. Companies such as Maidsafe, Storj, File Coin, and Sia all act as marketplaces. They offer quicker, simpler, and safer storage than DropBox, Amazon, or Google choices. Figure 9 shows the various categories for analyzing distributed cloud storage technologies.

6.1 MaidSafe

The SAFE (Secure Access For Everyone) Network is a P2P decentralized network of data and com- munications built by MaidSafe.net. The framework supports all existing centralized web applica- tions and data centers with a secure and confidential network of additional computing resources for their users. SAFE Network is a distributed, independent data storage and communications net- work [97]. The performance, reliability, privacy, and security of the network data are incredibly high. The current server-client-based internet offers information possession to whoever manages the servers, rather than to the data-creating people. The following categories are selected to ana- lyze the network.

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Table 1. Applications of Blockchain Technology for Cloud Storage

Authors Finding

Hoang Giang Do et al. [55]

Presented a BlockDS system for providing a reliable distributed storage service for searching keywords using blockchain technology.

Jingyi Li et al. [56] Suggested a deduplication scheme that allocates files to multiple servers and documents blockchain storage information. They describe smart contract-based protocols to ensure secure deduplication without central authority participants.

Bin Liu et al. [59] Suggested a framework for integrity service based on blockchain. This framework pro- vides a more trustworthy verification of the integrity of data for both consumers and data holders without depending on a trusted party.

Dongdong Yue et al. [60]

In P2P cloud storage, a blockchain-based method for verifying data integrity is intro- duced to make the verification process more effective and open. In this context, the Merkle tree is presented to identify data integrity and evaluate performance and relia- bility under various Merkle tree structures.

Josef Gattermayer et al. [63]

Proposed a multi-level system for clusters dependent on cryptocurrency principles to obtain a masterless reputation rating throughout the cluster.

Jiaxing Li et al. [66] Proposed a blockchain-based system for distributed cloud storage that would allow cus- tomers to divide their documents into encrypted chunks of information and upload them randomly to a peer-to-peer network.

Ilya Sukhodolskiy et al. [67]

Developed a model for controlling the access depending on blockchain, stored data in untrustworthy space, and implemented attribute encryption based on Ethereum smart contracts.

Jason Paul Cruz et al. [69]

An RBAC-SC was introduced to implement a trans-organizational framework for RBAC. Safe RBAC method (users cannot disguise roles and only allowed users must perform tasks), customer-oriented method (customers can report their duties to any agency), testable (everyone can verify the position of the user).

Yinghui Zhang et al. [72]

Introduced a BCPay, a cloud computing service payment system focused on blockchain. This system ensures that without any trusted party, the outsourcing services are paid safely and reasonably.

Changsong Yanga et al. [73]

Build a novel, data deletion scheme based on blockchain. If the server may not honestly delete the information, then this scheme allows the data owner to identify the cloud server’s malevolent operation.

Sharma et al. [86] The authors developed a novel architecture based on a decentralized blockchain cloud with an activated SDN controller at the end of the network to fulfill the essential design principles.

Hasan et al. [87] Developed an effective method to use blockchain and IFPS (InterPlanetary File System) to store provenance information. Users were also able to check the validity of their results.

Manzoor et al. [88] This study proposes a blockchain-based trading platform combined with a pairing free proxy re-encoding scheme to transfer sensor data to the user securely.

Zhang et al. [89] Development of a certificate less public validation technique against the procrastinat- ing auditor, namely, CPVPA, using on-chain currencies. Here, the verification process conducted is combined with the on-chain blockchain currency payment.

Wang et al. [90] Developed a decentralized scheme for securing cloud storage using access control. The conventional encryption algorithm based on attributes was modified by the introduc- tion of Ethereum’s smart contract technique. The distribution key is not reliant on the central authority, thus preventing the attacks on the central authority.

Zhang et al. [91] A secure technique of Public-key encryption with an option to search for keywords re- ferred to as SEPSE against keyword guessing attacks (KGA) is developed. In this scheme, users can encrypt keywords through a threshold with the help of specific key servers.

(Continued)

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Table 1. Continued

Authors Finding

Tosh et al. [92] A BlockCloud has been developed for the cloud computing framework, a blockchain- enabled information provenance architecture. Also, a PoS consensus mechanism was presented for BlockCloud to lessen the overhead of computational necessities that the conventional PoW consensus requires.

Chen et al. [93] They developed a framework for storing medical data by using blockchain and cloud storage technologies so that the data could be stored and shared safely.

Cao et al. [94] A secure eHealth framework assisted by the cloud referred to as TP-EHR, which ensured the integrity, correctness, and confidentiality of outsourced EHRs without the introduc- tion of a trusted entity was developed. The EHR generated by a doctor during the period of treatment was unified into the transaction of currencies based on blockchain. This technique employed a key agreement based on a user-friendly password for establish- ing channels among the doctors and the patients that are safe and can prevent attacks of guessing passwords without the need for any extra investments.

Wang et al. [95] A system that combined the attribute-based encryption (ABE), Ethereum blockchain, and decentralized storage system IPFS. The data holder can transmit the secret key to users and encrypt the shared information by specifying the access policy and the technique that has been done with fine-grained data access control.

Wang et al. [79] Developed protocol for cloud storage by utilizing blockchain and Rank-based Merkle AVL tree (RB-MHT) to accomplish preservation of privacy and batch auditing for main- taining the security of the modified record in the scheme based on blockchain.

Yu et al. [80] Developed a smart and decentralized public auditing plan for cloud storage, which elim- inated the requirement TPA for auditing.

Chen et al. [96] Suggested a performance-driven, auction-based reward system based on a blockchain consortium that ensures belief for both on-chain and off-chain information. Authors implemented a consortium that used as a distributed hyperledger to tackle on-chain data protection. Using an information performance-driven auction system, the assessed data performance used to maintain trust in off-chain data.

Distributed Storage: SAFE network resources are never higher than 1 MB each. Clients oper- ating with files greater than 1 MB would then have their data broken into 1 MB chunks, which will then be spread around a network. This specifies that a standard file consists of several parts: chunks that are individual portions of 1 MB after breaking the file and a datamap that stores each portion of the file’s identifier. The client maintains a datamap record of the resource identifier. Therefore, the entire file can be retrieved by first retrieving the particular resource (i.e., the datamap) even though it is distributed over many individual resources.

Consensus: Utilizing a method called Proof of Resource (POR), the system can validate in a mathematically verifiable manner which and what provides the resource. It is achieved by the network trying to save and access chunks of data on/from its nodes. A node’s ability to perform such activities will be calculated by a combination of its CPU power, availability of bandwidth, unused storage space, and online time.

6.2 Sia: Distributed, Blockchain-based Cloud Storage

Sia, a decentralized-storage network. Sia allows for the creation of peer storage contracts [98]. Contracts are treaties between a space supplier and their customer, defining what information will be contained and at what cost. Contracts are held in a blockchain, which makes them auditable publicly. Sia provides tenants with access to distributed cloud storage services to leverage cheaper, faster ways of using distribution centers accessible to anyone and are not regulated by a single authoritative source. Siacoin is based on a separate blockchain from Sia, and there are agreements between a storage renter and a supplier [99]. The selected categories are explained below.

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Table 2. Strengths of Works on Blockchain Technology for Cloud Storage

Work Confidentiality Integrity Authentication Access Control

Searching Auditing Blockchain-based distributed cloud

data storage

[55] � � � � � [56] � � � [59] � � � � [60] � � [63] � [66] � � � [67] � � � � [69] � � � [72] � � [73] � � � � [86] � � [87] � � � � [88] � � [89] � � � [90] � � � [91] � � [92] � � � [93] � � [94] � � � [95] � � � � [79] � � [80] � � �

Confidentiality: � -> privacy of information achieves through encryption and data access management. Integrity: � -> to manage unauthorized manipulation of data. Authentication: � -> to identify the valid user. Access Control: � -> to identify permission to use a resource. Searching: � -> process of locating data on the cloud. Auditing: � -> examination of records to validate data security. Blockchain-based cloud storage: � -> decentralized storage of cloud data without trusted parties.

Distributed Database: Upon uploading, the Sia program divides files into 30 parts, each tar- geted towards distribution to hosts worldwide. This distribution means no one host serves a single failure point and increases overall network uptime and redundancy.

Encryption: Every section of the file is encrypted before entering a renter’s machine. It means that only authenticated pieces of user data are stored in hosts. It is different from traditional cloud storage services like Amazon, which does not opt to encrypt user data. Sia utilizes the Twofish algorithm, an open-source and secure encryption standard [115].

Smart Contract: The renters form file contracts with hosts using the Sia blockchain. These contracts set rates, expectations on uptime, and other aspects of the renters-hosts partnership. The smart contract enables to create cryptographic Service-Level Agreements (SLAs), which are collected on the Sia blockchain. Since the network automatically enforces file contracts, Sia does not need intermediaries or trusted third parties.

Payment Method: Renters and hosts both use Siacoin, a special crypto-currency developed on the blockchain of Sia. Renters utilize Siacoin to purchase storage space from hosts, while hosts spend Siacoin as collateral in each file contract. After a file contract, the host must show that he/she

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Fig. 9. Identified categories for analyzing distributed technologies.

is saving the data for the renter. It is called proof of storage if the proof of storage appears on the blockchain within a given time, the host will be paid. If not, then the host will be penalized.

6.3 Storj

The Storj network is a reliable store of objects that encodes, fragments, and disperses data for storage to nodes worldwide. Information is kept and delivered in a way intended to prevent viola- tions. The platform that underlies Storj is a peer-to-peer, implementable space contract. It is a way for two entities (or computers) to decide to trade a certain amount of storage for money without knowing each other. They call the machine-selling space the “farmer,” and the computer-buying space, the “renter.” Storj implements encryption on the client-side, which means that only the person who uploads the file has access to that. Sharding is used for the splitting of data between several nodes [100]. For decentralized networks, this provides stability, preserving connectivity even when nodes fall off the network. Also, if a network host could decrypt a file, then they would have only one small piece. Hosts are “audited” continuously with an automated algorithm that verifies that they have the documents that they claim to have.

Distributed Hash Table (DHT): A DHT is simply a means of transforming a bunch of self- organized nodes into a functional web. The DHT enables farmers and tenants to transmit their contract offers to a large node community rather than having a central server register each node and manage all contracts.

Contracts and Audits: A fixed-term contract. Over this period, the renter keeps an eye that the farmer is still available. The farmer responds with encoded evidence that the file is still in them. The renter ends up paying the farmer for every proof they get and verify. This method of challenge-> evidence-> payment is called an “audit,” since the renter audits the farmer’s storage.

6.4 Swarm: Serverless Hosting Incentivized Peer-to-Peer Storage

and Content Distribution

Swarm is a decentralized storage network and content delivery tool, the ethereum web3 stack’s native base layer service. Swarm’s main objective is to provide the public record of Ethereum with a sufficiently decentralized and robust archive, in particular, to store and distribute Decentralized

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Web Applications (dapps) code and data and blockchain data. From an economic perspective, it allows users to pool their storage and bandwidth resources to provide those utilities to all members of the network. At the same time, Ethereum encourages them to do so. Swarm [101] offers a peer-to-peer application and service solution that is fault-tolerant, surveillance-resistant, and auto-sustaining and facilitates transaction exchange resources. The following selected categories are summarized below.

Distributed Database: Uploading data comprises of posting information at local Swarm node, followed by “synchronizing” local Swarm node to the resulting chunks of data with their peers in the network. In the meantime, downloading content comprises of local Swarm node querying the appropriate chunks of data for its peers in the network and then reconstructing the information locally.

Encryption: The encryption feature is intended to protect the data and make the shredded information hard to read for any Swarm node handling. To encrypt and decrypt the content, Swarm uses counter mode encryption. When uploading content to Swarm, the uploaded data is broken down into 4 KB chunks. These chunks are all encoded with a separate encryption key created at random.

6.5 Filecoin

Filecoin is a distributed platform [102] that transforms the cloud storage over an algorithmic sector. The platform executes on a blockchain with a proprietary token (also called as “Filecoin”), which miners are obtaining by supplying space for storage to clients. Instead, customers are spending Filecoin employing miners to save or circulate data. Filecoin functions as a reward layer on top of IPFS [103] that can provide any data storage infrastructure.

Decentralized Storage Network (DSN): Filecoin DSN is a distributed storage system that can be audited, publicly validated, and incentively built. Customers pay a network of miners for information storage and retrieval; in return for fees, miners provide disk space and bandwidth. DSNs cumulative repository provided by numerous individual space suppliers and self-contained, providing customers with data storage and access.

Consensus: Storage providers have to persuade their customers in the Filecoin protocol that they saved the information they were paying to store; in practice, storage providers must create Proofs-of-Storage (PoS) that the blockchain network (or the customers themselves) verifies. Proof- of-Replication (PoRep) is a novel Proof-of-Storage that enables a server to persuade a customer that certain information has been repeated into its own unique physical space. Proof-of-storage systems allow customers to verify whether a storage provider is storing the outsourced data at the moment of the challenge.

Smart Contract: Filecoin offers end-users with two core primitives: Get and Put. These primitives enable customers to store information at their preferred price and to retrieve data from the markets. In contrast, the primitives provide the standard use cases for Filecoin by promoting smart contracts.

6.6 IPFS: InterPlanetary File System

The IPFS is a decentralized, peer-to-peer file network designed to link all computer nodes to the same file system. IPFS integrates a distributed hash table, a block sharing reward, and a self- certification namespace. IPFS has no single point of fault, and nodes do not need to believe each other. IPFS is a P2P document sharing method aimed at radically changing the way information is shared across the globe. IPFS [103] is made up of a variety of developments in communication protocols and decentralized systems combined to create a file system like no other. Nodes can store and share data with the Distributed Hash Table without central coordination. The following categories are selected for analyzing the IPFS network.

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Table 3. Building Blocks of Distributed Cloud Storage Technologies

MaidSafe Sia Storj Swarm Filecoin IPFS

Compensation Model

Payment per storage space, CPU, bandwidth, and online time

Determined by a documented contract between a storage renter and a provider

The platform currently pays storage providers and bills storage renters monthly

Decentralized storage network and content delivery tool

Peer-to-Peer storage marketplace built on IPFS

Data blocks bartered with reciprocating file sharing (Bitswap)[119]

Who Provides Payments?

Users and generated tokens

Storage renter Storage renter (via Storj)

Built-in incentive Storage renter (via Filecoin)

File downloaders

Blockchain Foundation

None uses close groups consensus

Independent Sia blockchain

Counterparty bitcoin blockchain

Web3 ethereum stack

Blockchain None uses bitswap credit protocol

Target Use cases and Scenarios

Encrypted cloud storage, web hosting, streaming

Encrypted cloud storage

Encrypted cloud storage

Messaging, data sharing, peer-to-peer payment portals, and storage facilities

Encrypted cloud storage

File hosting, versioning, web hosting, and content distribution

Distributed Hash Table: A hash table is a set of information that gathers data as key/value sets. Information is distributed over a network in DHT and organized effectively to allow fast access and lookup between nodes. Decentralization, fault tolerance, and scalability are the key benefits of DHTs.

Merkle DAG: A Merkle DAG is a combination of a Merkle tree and a Directed Acyclic Graph. Merkle trees guarantee right, undamaged and unaltered data blocks exchanged over P2P networks. This test is done by using hash algorithms to group data blocks. Hash is primarily a function that takes up the input and generates a single alphanumeric (hash) sequence, which refers to that input. Table 3 highlighted the basic building blocks of distributed technologies.

7 PROPERTIES OF BLOCKCHAIN FOR CLOUD STORAGE

• Immutable: (tamper-proof and permanent) the blockchain is generally a permanent record for the transactions. Hence there is no provision to alter the block once added, thus creating trust within the record of the transaction [104].

• Decentralized: The blockchain divides the data into little pieces and circulates them while uploading it into the cloud server. A decentralized scheme offers a wide range of benefits over the more traditional centralized schemes, including increased system reliability, scale, and privacy. Hence, a file stored in the blockchain can be copied or accessed by any node of the network. In a decentralization network, each cloud hosting server is accountable for their cloud environment and could interact and collaborate via the blockchain network with other providers. Blockchain technology is a distributed platform with a secure and dis- tributed ledger for cloud manufacturing [105]. The blockchain is primarily a digital ledger of transactions, since the computer has a complete copy of the ledger, so there is no result in the data loss. Blockchain’s digital ledger technology retains integrity and confidentiality, lowers the cost of computing, and enhances precision. Further, the transaction process is secured, and no other third party can access the transaction using blockchain technology.

• Data Validation and Encryption: As secret commercial information is shared on cloud servers, it is essential to implement an effective encryption method to encode outsourced data. Blockchain encodes everything, and it is probably going to demonstrate that informa- tion has not been altered. Furthermore, the users can check or verify the file signature that has been changed or not while distributing the data. If someone tries to change or alter the

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file, then the file signature will be invalid. No one can deny that blockchain offers a reliable and secure solution with independent information confirmation.

• Reliable Service: Blockchain is a representative anonymity technology. Blockchain can be upgraded to a reliable service in combination with the cloud computing environment [107]. The blockchain makes it unfeasibly hard for attackers to obtain and split network data from a storage process. The blockchain’s information is distributed, encrypted, and cross-checked.

• Obscurity Empowering: The public blockchains have gotten the early features and ap- provals for empowering obscurity, making private blockchains that confine access to spe- cific clients. Regardless of this, we understand the advantages of a decentralized distributed system. Yet, anybody getting to a private blockchain must validate their personality to ob- tain entrance benefits, which tend to be limited to specific transactions, thereby increases privacy protection [108].

• Automation through smart contracts: One of the most likely consequences of blockchain technology is companies and individuals’ potential to cut off the intermediaries in terms of information monitoring. However, one blockchain-related technology that would improve how people make a trade is smart contracts, which can potentially automate all transactions and exchanges.

• Verifiability: Blockchain not only stores data in a distributed and authenticated manner but also offers a serial chain with a cryptographic hash of the block in each transaction for the verification purpose. It connects the blocks, creating a decentralized and tamper-proof transaction ledger.

8 INTEGRATING BLOCKCHAIN WITH CLOUD STORAGE-REQUIREMENTS

AND CHALLENGES

Blockchain integration with cloud [109] gathers massive virtualized service structures, including hardware and software tools. Within the sector, these systems are referred to as “Infrastructure as a Service” (IaaS), “Platform as a Service” (PaaS), and “Software as a Service” (SaaS). Cloud computing services are supported in large data centers, sometimes referred to as “data farms.” Public clouds offer limitless access to shared information and assets for a broad range of clients, yet there is no assurance that clients’ information will be secured. Accessing the data and resources in private clouds is confined, and there is a need for all the users to be validated with the help of strong au- thentication and authorization processes. Generally, the enterprises are the owners of the private cloud clusters and work under explicit cloud standards. Hybrid clouds appear to be a perfect model for integrating numerous private clouds into a combined global framework. Such incorporation is done via the public layer of the upper level. The main issue with that model is to reach an un- derstanding among private cloud providers for operating under the unified public cloud standard. A considerably more realistic situation is the “many cloud model,” in which the delegated private cloud groups are connected by using the regular P2P organize (Figure 10).

It tends to be seen that an identical model is suitable for blockchain that was the primary expla- nation behind attempting to incorporate the two conditions to improve the security approaches in cloud environments. There are two principle strategies for integrating the cloud and blockchain platforms.

• Utilizing cloud for the advancement of blockchain applications and aiding the coordination with large business systems (private clouds) encourage replication, storage, and transac- tional data access.

• Utilizing blockchain strategies for improving client security, managing the information, and task in the cloud.

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Fig. 10. The architecture of “Many Clouds” based on P2P.

For blockchain systems, the transaction number can be massive. The enormous amounts of information created need flexible resources in information handling. Scalability and Flexibility are perhaps the most critical functionalities within the cloud frameworks to provide dynamically changing activities with on-demand cloud properties. Public clouds have the potential to deliver a large-scale resource network that is open to consumers who pay only for those that are being used. More often than not, private clouds should be configured to accommodate enormous datasets. Cloud systems can effectively cover the physical area of knowledge from a security perspective. Tuning experiments can be continuously performed with a negligible impact on deployed appli- cations, which is crucial to the successful implementation of most blockchain algorithms. Any blockchain system must accept data sovereignty rules and store and process the information only in areas allowed by the guidelines. This means that the cloud service provider requires its cus- tomers to have control over the areas in which their information is processed and stored. Another significant problem with blockchain systems is the resilience of the architecture and the power to a fault. This means that even a single node failure in the blockchain network should not af- fect the entire framework’s work. In these instances, cloud services aid by replicating the stored information and using different programming applications.

At last, using the algorithms of the blockchain might enhance the blockchain framework se- curity. Within a centralized cloud environment, the program could be managed centrally with information being stored on the local data server. Oracle Blockchain Cloud Service venture [110] and iEx.ec venture [111] are ongoing instances of such effective coordination of the blockchain with cloud stages.

The mapping of blockchain technology features with the safety services supplied to protect cloud information is presented in Table 4. This table shows the case where we can take advantage of the numerous blockchain features to ensure cloud data privacy, confidentiality, accessibility, and transparency, thus increasing efficiency and accuracy.

9 SOME IMPORTANT INFERENCES AND RECOMMENDATIONS

• Identity Management System: The identity management scheme is vital for CSP and cloud computing users. The users adopt their identity to access their data in the cloud. Different limitations of the identity administration system are recognized. Blockchain technology provides a means to avoid this issue by providing a safe method without any trustworthy

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Table 4. Mapping of Blockchain Characteristics with Security Services

Secure Transactions Case (Digital Ledger Technology)

Safe Data Storage Case (Decentralized)

Authentication Case (Data validation and encryption)

Minimizing Error Case

Secure Blockchain Solution

Integrity � � Scalability � � � Confidentiality � � � � � Anonymity � � Efficiency � � � � Computation Cost � � � Privacy Protection � � � Availability � � � �

party. It could be used to create a blockchain-based identity system, making it possible for entities to handle it, offering them more power over who has their data and how they access it. It is, therefore, necessary to combine the decentralized blockchain principle with identity control to create a digital ID that will function as a digital watermark that can be given to any internet transaction [112].

• Secure Data Classification: Cloud computing data centers can store the data of distinctive users. Based on the importance of information, the characterization of information provides security in the cloud. Dependent on the category of data, this scheme offers various aspects such as recurrence, refresh recurrence, and access by different users [113]. The security level incorporates classification, encryption, respectability, capacity, and so on that are chosen to depend on the type of information [114]. The classified information must be secured, since it contains the most sensitive information. There is a definite need for a better solution based on blockchain security methods to secure a customer’s classified information.

• Cloud Computing Service for Trust-based Secure Solution: The security domain for im- plementation of the cloud computing administrations alongside in general security contem- plations is a test. The cloud computing framework should be focused on the requirement of a secure and trusted solution based on blockchain for cloud computing service [115]. Blockchain is a concept aimed at decentralization as a security measure, has the function of creating a global index for all transactions that take place in a given network and makes them unchangeable [116].

• Infrastructure Security: Concerning which party (customer or CSP) provides which pro- tection methods there is a need for transparency. There is likely a need for agreement, which gives the details about the security capacities and more noteworthy confirmation over the CSP’s efforts and capabilities [117]. To address the issues, the identity administra- tion system needs to be adopted in the current blockchain infrastructure for resolving the interrelationship between service model, user, and system [118].

• Data Storage and Security: For the future of cloud computing, information security abil- ities are significant. This allows us to save information in the cloud and be safe. For data storage and security in the future, they combined with the present deficient encryption, key administration capacities, and cryptographic research endeavors. For example, to restrain the amount of information, the predicate encryption can be used to decode the information in the cloud [119]. Cloud computing’s main significance is homomorphic encryption, which involves the process of encoding information in the cloud. Therefore, the future business

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Blockchain Technology for Cloud Storage: A Systematic Literature Review 89:25

Table 5. Mapping of Research Questions with the References

Research Question Reference

RQ1 [3, 4, 6, 7, 14, 15, 37, 38, 39, 61, 65, 109]

RQ2 [1, 8, 9, 28, 29, 30, 31, 32, 33, 34, 35, 36, 114, 115]

RQ3 [5, 13, 28, 29, 30, 40, 54, 57, 58, 106, 116, 117, 131]

RQ4 [2, 10, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 41, 42, 43, 44, 45, 46, 47, 52, 53, 59, 70, 71, 75, 76, 93, 96, 97, 98, 104, 107, 108, 122, 123, 124, 125, 126, 128, 129, 130, 132, 133, 134, 135]

RQ5 [11, 12, 48, 49, 51, 55, 56, 60, 62, 63, 67, 68, 69, 72, 73, 74, 77, 78, 79, 81, 82, 83, 85, 86, 90, 91, 92, 93, 94, 95, 99, 100, 101, 102, 103, 105, 110, 111, 121]

RQ6 [48, 49, 50, 51, 60, 62, 63, 64, 66, 72, 74, 77, 78, 79, 80, 82, 83, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 99, 100, 101, 102, 103, 112, 113, 118, 120, 127]

practicality of such abilities would be a huge advantage to cloud computing [120]. It needs a block-based system with a secure boolean search for secure cloud storage.

• Security Management: There is a need to initiate standard organizations (e.g., World Wide Web Consortium (W3C), the Internet Engineering Task Force (IETF), the Organization for the Advancement of Structured Information Standards (OASIS)) and start new efforts for the management protocols that interoperate with many clouds [121, 122]. To increase cloud selection, the cloud management standards will be created and supported by the cloud ser- vice provider, which encourages consistent interoperability crosswise over different clouds. Like the customer/server period, guidelines will make a biological system of Independent Software Vendors (ISVs) and specialist organizations that give clients decision, adaptability, and greater agility by the method of computerization [123]. Also, there is a need for Soft- ware Defined Networking (SDN)-based blockchain to adapt the security without the review of the administrator automatically. It can alleviate specific problems such as adaptability, effectiveness, accessibility, and safety [124].

• Privacy: Privacy is essential for the CSP to understand the main privacy laws to realize the data transaction from one place to another. It is yet to solve the type of government intercession or the formation of a worldwide security standard that will give reliability crosswise controls. Specific gauges will offer to characterize the way organizations can use cloud computing [125]. When cloud computing turns out to be more standard, the reports of regular review increased by specific necessities around protection and security may review concerned with the handling of information and its protection concerns [126]. Blockchain- based data provenance architecture is necessary to ensure the processing of information in cloud storage while at the same time, increasing privacy and accessibility [127].

Table 5 summarizes the details of the studies addressing each research question.

10 CONCLUSIONS

In this article, a systematic survey (2010–2019) of blockchain technology and cloud computing to secure cloud storage has been assessed. Blockchain presents numerous guarantees for the future of cloud data. The first one is that clients could be responsible for controlling their data and the transactions in various areas. Hence, trust is built that the transactions are being executed precisely as per the protocol directions, eliminating the trusted third-party requirement. This idea can impact cloud data to discover a solution for managing and storing the information appropriately on a P2P network. Blockchain innovation can be another piece of the encompassing biological system of instruments that cloud data utilizes. In reality, it can assume a vital job in security for authenticating and preventing access depending on the user’s requirements by

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89:26 P. Sharma et al.

recording the histories of data access and legitimate utilization of encryption on the information. A few challenges are still present, for example, accord models, the computational expenses of mining blocks, and transaction validation. Additionally, Blockchain applications provide solutions involving essential changes or complete replacement of existing frameworks. That is the reason for the transaction not being quick and straightforward. In any case, we are still in the beginning stages of the development of Blockchain, and these impediments will, in the end, be overcome, opening the way for some energizing potential outcomes.

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Received October 2019; revised February 2020; accepted May 2020

ACM Computing Surveys, Vol. 53, No. 4, Article 89. Publication date: August 2020.

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